Pinhua Xie

A paradox for air pollution controlling in China revealed by “APEC Blue” and “Parade Blue”

A series of strict emission control measures were implemented in Beijing and surrounding regions to ensure good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit and 2015 Grand Military Parade (Parade), which led to blue sky days during these two events commonly referred to as “APEC Blue” and “Parade Blue”. Here we calculated Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) and Ozone Monitoring Instrument (OMI) NO2 and HCHO results based on well known DOAS trace gas fitting algorithm and WRF-Chem model (with measured climatology parameter and newest emission inventor) simulated trace gases profiles. We found the NO2 columns abruptly decreased both Parade (43%) and APEC (21%) compared with the periods before these two events. The back-trajectory cluster analysis and the potential source contribution function (PSCF) proved regional transport from southern peripheral cities plays a key role in pollutants observed at Beijing. The diminishing transport contribution from southern air mass during Parade manifests the real effect of emission control measures on NO2 pollution. Based on the ratios of HCHO over NO2 we found there were not only limited the NO2 pollutant but also suppress the O3 contaminant during Parade, while O3 increased during the APEC.

Impacts of Photoexcited NO2 Chemistry and Heterogeneous Reactions on Concentrations of O3 and NOy in Beijing,Tianjin and Hebei Province of China

Nitrous acid (HONO) plays a significant role in the photochemistry of the troposphere, especially in the polluted urban atmosphere, due to its photolysis by solar UV radiation into the hydroxyl radical (OH), which is one of the most important oxidant in the atmosphere (Alicke et al., 2002). Some previous observations showed unexpected high HONO concentrations up to several ppb at urban or rural sites during the daytime or nighttime (Qin et al., 2009; Su et al., 2008a, 2008b; Yu et al., 2009) but gas-phase chemical models usually underestimated HONO observations, particularly in the daytime. HONO sources are thought to be direct emissions, homogeneous gas reactions, and heterogeneous reactions on aerosol surfaces. Sarwar et al. (2008) incorporated gas-phase reactions, direct emissions, a heterogeneous reaction, and a surface photolysis reaction into the CMAQ model, and simulations still indicated HONO underestimation by comparison with measurements, especially in the daytime. Li et al. (2008) suggested a reaction of electronically excited nitrogen dioxide (NO2*) with water vapor as follows,

Emission Flux Measurement Error with a Mobile DOAS System and Application to NOx Flux Observations

Mobile differential optical absorption spectroscopy (mobile DOAS) is an optical remote sensing method that can rapidly measure trace gas emission flux from air pollution sources (such as power plants, industrial areas, and cities) in real time. Generally, mobile DOAS is influenced by wind, drive velocity, and other factors, especially in the usage of wind field when the emission flux in a mobile DOAS system is observed. This paper presents a detailed error analysis and NOx emission with mobile DOAS system from a power plant in Shijiazhuang city, China. Comparison of the SO2 emission flux from mobile DOAS observations with continuous emission monitoring system (CEMS) under different drive speeds and wind fields revealed that the optimal drive velocity is 30–40 km/h, and the wind field at plume height is selected when mobile DOAS observations are performed. In addition, the total errors of SO2 and NO2 emissions with mobile DOAS measurements are 32% and 30%, respectively, combined with the analysis of the uncertainties of column density, wind field, and drive velocity. Furthermore, the NOx emission of 0.15 ± 0.06 kg/s from the power plant is estimated, which is in good agreement with that from CEMS observations of 0.17 ± 0.07 kg/s. This study has significantly contributed to the measurement of the mobile DOAS system on emission from air pollution sources, thus improving estimation accuracy.

Variation of temporal and spatial patterns of NO2 in Beijing using OMI and mobile DOAS

Control policies such as “odd-and-even license plate rule” were implemented by the Chinese government to restrict traffic and suspend factory production in Beijing and neighboring cities during the Asia-Pacific Economic Cooperation summit. We use ozone monitoring instrument (OMI), mobile differential optical absorption spectroscopy (DOAS), and multi-axis differential optical absorption spectroscopy (MAX-DOAS) to measure the variation of the spatial and temporal patterns of NO2 column densities from October 24, 2014 to November 22, 2014. It is found that the NO2 column densities during the episode of control policies are significantly lower than those during other periods, and the emission flux of NO2 calculated by mobile DOAS is also lower than the results from other periods. Some daily low NO2 column densities occur with the northwest wind direction. We then compare the relationship between OMI and mobile DOAS NO2 column density observations, and the results of mobile DOAS are approximately 2.7 times larger than the OMI values. The largest discrepancy occurs in the northern part of Beijing city. In other parts, the two instruments have a better correlation coefficient (R2) of 0.61. The low NO2 column densities that occur during the episode of control policies are affected by the control policies as well as meteorological conditions.

Retrieval of ultraviolet skylight radiances and O3 slant column densities from balloon-borne limb spectrometer

With a novel light-weight and absolutely calibrated ultraviolet (UV) spectrometer, UV skylight radiances and O3 slant column densities are measured by balloon-borne limb measurements in Xinjiang area, China. UV skylight radiances measured at the height of 31 km are compared with the results from Modtran in the wavelength range from 290 to 420 nm. O3 slant column densities are derived from radiance spectra in the Huggins bands (320-335 nm) using differential optical absorption spectroscopy method. And the parameter exhibits a good correlation with the same value simulated by radiative transfer model (Tracy). The O3 profile simultaneously measured by an O3 sonde is used as input in Tracy calculations. The O3 sonde is launched on the same balloon.

Observation of tropospheric NO2 by airborne multi-axis differential optical absorption spectroscopy in the Pearl River Delta region, south China

An airborne multi-axis differential optical absorption spectroscopic (AMAX-DOAS) instrument was developed and applied to measure tropospheric NO2 in the Pearl River Delta region in the south of China. By combining the measurements in nadir and zenith directions and analyzing the UV and visible spectral region using the DOAS method, information about tropospheric NO2 vertical columns was obtained. Strong tropospheric NO2 signals were detected when flying over heavilly polluted regions and point sources like plants. The AMAX-DOAS results were compared with ground-based MAX-DOAS observations in the southwest of Zhuhai city using the same parameters for radiative transport calculations. The difference in vertical column data between the two instruments is about 8%. Our data were also compared with those from OMI and fair agreement was obtained with a correlation coefficient R of 0.61. The difference between the two instruments can be attributed to the different spatial resolution and the temporal mismatch during the measurements.

Effect of Atmospheric Interfering Absorption on Measurement of BTX by DOAS

It was reported on the elimination of interfering absorption of BTX. the absorption of O2 includes different absorption bands, which change differently when the partial pressure of oxygen is varied. These cause the nonlinear absorption of O2 and the observed band shape to vary with the column density of O2. The absorption ratios of molecular absorption in each of the Herzberg bands and dimer absorptions, as well as the contribution to the correction error of molecular absorption, are studied based on the characteristic of these absorption bands. The optimized way to eliminate the interfering absorption is obtained in the end and the effectiveness of using interpolation proposed by Volkamer et al. to remove O2 absorption is proved again. As to O3 and SO2, the effect of the thermal effect of characteristic spectra on the elimination error of their absorption is studied. Solutions to these problems are discussed and demonstrated together with methods to optimize the interpolation of spectra. As a sample application, differential optical absorption spectroscopy (DOAS) measurements of BTX are carried out. Results show a low detection limit and the good correlation with point instruments are achieved. All these prove the feasibility of using spectral interpolation to improve the accuracy of DOAS measurements of aromatic hydrocarbons for practical purposes.

Preflight Evaluation of the Performance of the Chinese Environmental Trace Gas Monitoring Instrument (EMI) by Spectral Analyses of Nitrogen Dioxide

The Environmental trace gas Monitoring Instrument (EMI) onboard the Chinese high-resolution remote sensing satellite GaoFen-5 is an ultraviolet–visible imaging spectrometer, aiming to quantify the global distribution of tropospheric and stratospheric trace gases and planned to be launched in spring 2018. The preflight calibration phase is essential to characterize the properties and performance of the EMI in order to provide information for data processing and trace gas retrievals. In this paper, we present the first EMI measurement of nitrogen dioxide (NO2) from a gas absorption cell using scattered sunlight as the light source by the differential optical absorption spectroscopy technique. The retrieved NO2 column densities in the UV and Vis wavelength ranges are consistent with the column density in the gas cell calculated from the NO2 mixing ratio and the length of the gas cell. Furthermore, the differences of the retrieved NO2 column densities among the adjoining spatial rows of the detector are less than 3%. This variation is similar to the well-known “stripes-pattern” of the Ozone Monitoring Instrument and is probably caused by remaining systematic effects like a nonperfect description of the individual instrument functions. Finally, the signal-to-noise ratios of EMI in-orbit measurements of NO2 are estimated on the basis of on-ground scattered sunlight measurements and radiative transfer model simulations. Based on our results, we conclude that the EMI is capable of measuring the global distribution of the NO2 column with the retrieval precision and accuracy better than 3% for the tested wavelength ranges and viewing angles.

MAX-DOAS observations and their application to the validation of satellite and model data in Wuxi, China

From 2011 to now a MAX-DOAS instrument developed by the Anhui Institute of Optics and Fine Mechanics institute is operated in Wuxi, China, which is along the Yangtze River. We determine the tropospheric vertical column densities (VCDs), near surface concentrations and vertical profiles of aerosols, NO2, SO2, HCHO from the MAX-DOAS observations using the DOAS method and profile retrieval algorithm (refered to as “PriAM”). We verified the results by comparing them with other independent techniques, such as sun photometer (Aeronet), a visibility meter and a long-path DOAS instrument. We acquire the cloud and aerosol conditions using a cloud classification scheme based on the MAX-DOAS observations. Based on the obtained results, we characterize the effect of the clouds on MAX-DOAS. Then we characterize the diurnal, annual and weekly variations of the trace gases and aerosols and validate the tropospheric trace gas VCDs derived from the Ozone Monitoring instrument (OMI) on the Aura satellite platform as well as the simulations from the IMAGES, CHIMERE and Lotos chemical transfer models. We analyse the agreement between MAX-DOAS, satellite and model datasets in different sky conditions. Besides the direct comparison with the satellite data, we also use the trace gas and aerosol profiles derived from MAX-DOAS to recalculate the air mass factor (AMF) for the satellite observations and to evaluate the corresponding improvement of the satellite VCDs. In some periods with strong aerosol pollution, we evaluate the effect of the aerosols on the satellite tropospheric AMF of the trace gases. Here should be noted that the aerosol effect on the AMF is not yet considered in the published satellite products, which cause appreciable errors of the tropospheric VCD of satellite products around polluted regions.

Temperature Dependence of Atmospheric NO3 Loss Frequency

A new indicator with temperature dependence of the NO3 loss frequency, was developed to study the contribution of NO3 to the oxidation of monoterpenes and NOx removal in the atmosphere. The new indicator arises from the temperature dependence of kinetic constant. The new indicator was applied to data of observation based on differential optical absorption spectroscopy system on the outskirts of Hefei, China. According to the findings, the contribution of monoterpenes to the loss of NO3 was 70%–80%.